As noted in Cotton and Pielke (1995) the dominant process for
precipitation formation in warm clouds is collision and coalescence.
We have seen that this process is very effective in clouds which are
warm-based and maritime, or have substantial liquid water contents.
The collision and coalescence process among liquid drops is also an
important contributor to rain formation in many mixed-phase clouds, and
the presence of supercooled drizzle-drops and raindrops enhances the
rate of formation of precipitation in supercooled portions of clouds
as well.

One method of seeding clouds to enhance precipitation is to
introduce hygroscopic particles (salts) which readily take on water by
vapor deposition in a supersaturated cloudy environment. The
conventional approach is to produce ground salt particles in the
size-range of 5-100 , and release these particles into the
base of clouds. These particles grow by vapor deposition and readily
reach sizes of 25 to 30 in diameter or greater. They are
then large enough to serve as ``coalescence'' embryos and initiate or
participate in rain formation by collision and coalescence.

Cotton and Pielke (1995) reviewed the various physical and
statistical experiments that have been carried out over the years. The
results of the statistical experiments were generally inconclusive
though some suggested positive effects. Observational and modeling
studies provide further support that at least in some
clouds, the addition of hygroscopic seeding material can broaden
drop-spectra and at least hasten the onset of precipitation formation.
We concluded that `` there appears to be a real opportunity to enhance
rainfall through hygroscopic seeding in some clouds. It has not been
determined how open the `window of opportunity' actually is. In
warm-based, maritime clouds the rate of natural production of rainfall
may be so great that there is little opportunity to beat nature at its
own game. On the other hand, some cold-based continental clouds may
have so many small droplets that seeding-produced big drops cannot
collect them owing to very small collection efficiencies. Thus there
probably exists a spectrum of clouds between these two extreme types
that have enough liquid water to support a warm cloud precipitation
process that can be accelerated by hygroscopic seeding. The problem is
``to identify those clouds, and deliver the right amount of seeding
material to them at the right time.''

As optimism for significant precipitation enhancement by static
seeding of supercooled clouds has waned, enthusiasm for the potential
of hygroscopic seeding has grown. Two ongoing research programs, one in
Thailand, the other in South Africa, have contributed to that
enthusiasm.

The South African experiment was motivated by a report by
Mather (1991) which suggested that large liquid raindrops at -10C found
in a cumulonimbus were the result of active coalescence processes
caused by the effluent from a Kraft paper mill. Earlier, Hobbs et al. (1970)
found that the effluent from paper mills can be rich in cloud
condensation nuclei (CCN). Moreover, Hindman et al. (1977a,b) found
paper pulp mill effluent to have high concentrations of large and
ultra-giant hygroscopic particles, which is consistent with the idea
that the paper pulp mill effectively ``seeded'' the storm.

Another reason for optimism is that Mather et al. (1996b) applied a
pyrotechnic method of delivering salt, based on a fog dispersal method
developed by Hindman (1978). This reduced a number of technical
difficulties associated with preparing, handling, and delivery of
very corrosive salt particles. Seeding with this system is no more
difficult than silver iodide flare seeding. Compared to conventional
methods of salt delivery, the flares produce smaller-sized particles
in the size range of 0.5 to 10 . Thus, not as much mass
must be carried to obtain a substantial yield of seeding material. The
question of effectiveness of this size range will be discussed below.
Seeding trials with this system suggested that the pyrotechniques
produced a cloud droplet spectrum that was broader and with fewer
numbers, which would be expected to increase the chance for initiation
of collision and coalescence processes.

Mather et al. (1996b) analyzed radar-defined cells over a period of about an
hour to identify the seeding signatures for 48 seeded storms compared
to 49 unseeded storms. They showed that after 20 to 30 minutes, the
seeded storms developed higher rain masses and maintained those higher
rain masses for another 25 to 30 minutes. Bigg (1997) performed an
independent evaluation of the South African exploratory hygroscopic seeding
experiments and also found that the seeded storms clearly lasted
longer than the unseeded storms. Bigg also suggested that there was a
clear dynamic signature of seeding. He argued that hygroscopic seeding
initiated precipitation lower in the clouds, which, in turn, was not
dispersed horizontally as much as the unseeded clouds by vertical wind
shears. As a result,
Bigg speculated that low-level downdrafts became more intense, which
yielded stronger storm regeneration by the downdraft outflows, and
longer-lived precipitation cells.

Biggs hypothesis is a plausible scenario that should be examined
thoroughly with numerical models and coordinated, high resolution
Doppler radars.

Cooper et al. (1997) performed simulations of the low-level evolution
of droplet spectra in seeded and unseeded plumes. Following a parcel
ascending in the cloud updrafts they calculated the evolution of
droplet spectra by vapor deposition and collection. The calculations
were designed to emulate the effects of hygroscopic seeding with the
South African flares. The calculations showed that introduction of
particles in the size-range characteristic of the flares resulted in
an acceleration of the collision and coalescence process. If the
hygroscopic particles were approximately 10 in size,
precipitation was initiated faster. But, when more numerous 1
hygroscopic particles were inserted, high concentrations of
drizzle formed. For a given amount of condensate mass, if the mass is
on more numerous drizzle drops than on fewer but larger raindrops,
then evaporation rates are greater in the subcloud layer. This could
lead to more intense dynamic responses as proposed by Bigg, suggesting
that seeding with smaller hygroscopic particles may have some
advantages. Keep in mind, however, that this is a very simple model.
More comprehensive model calculations should also be performed.

In summary, there are some exciting new results of hygroscopic seeding
with flares. This work is still very exploratory and is a long way
from proving that such techniques can make significant increases in
rainfall on the ground for a variety of weather and climate regimes.
It is refreshing for a change to end an overview of the science of weather
modification by cloud seeding on a rather upbeat note!